Can I charge credit cards while I’m on a call?

Always yes if you’re on Wi-Fi. If not, it can depend on both your
carrier and your connection; the key is “simultaneous voice and
data”. High quality AT&T connections support this (labeled 3G or
4G), but the older Edge and GPRS networks do not. Verizon and Sprint
do not support simultaneous voice/data on the iPhone but do for some
Android phones.